The timestamp in the exported frames file is the reception time of the frame as set by the ground station. From my experience, however not fully researched and analyzed, the accuracy of the timestamp may vary from some (milli-)seconds to a couple of minutes in SatNOGS Network stations, unfortunately I have no idea for 3rd party observers.
Unfortunately for some months we had an issue that affected some Network’s stations and made them to send frames with wrong timestamps (not UTC time but local time timestamp). You can read more about it in Important bug-fix for SatNOGS Network stations with version >2.0.0. Maybe this could be the reason of seeing frames out-of-order.
With more details, for example from which stations you got out-of-order data, is this consistent etc, we may be able to analyze this situation and find out what causes this problem.